Exhaust Fan Venting - Bathroom, ERV, & HRV

In another thread (that grew quite unwieldy) the question came up about exhaust venting

Robert Riversong: "the most foolish way to duct an exhaust fan is straight up through the roof"

John Brooks: "It seems to me that in a heating climate (or during the heating season)... It would be desirable to have the "passive NPP" high. This would make the house more "negative" and would reduce (or eliminate) positive pressure near the top of the enclosure."

At this time I brought up the issue with condensation, wind, and dampers & that they should be vented out through a gable end if possible, if not then the roof but never through a soffit. This was replied to with a "I did say "During the Heating Season"" so I thought maybe we should take a look at this

First, condensation is a big concern during the "Heating Season" as these fans are transporting humid warm air up & passing through a super cold area

Wind is a big issue which is one reason why chimneys have to be vented so high

The final issue is the dampers, if one was to get stuck open, you would have a nice big hole or open chimney transporting all that nice conditioned hot air straight out of the house 24x7.

Replies to This Discussion

My take is this, vent exhaust fans through the roof (if gable venting is not a practical option). Short runs, insulate the duct(s), and buy quality vent caps. This post also reminds me of the importance of regular follow-ups with clients for whom work has been performed for.

Thanks for popping in John & forgive me for saying this but this is the "Best Practices" group & exhaust only ventilation & uncontrolled ventilation are definitely not a best practice which is why I labeled it the way I did. Now in some cases we might have to live with it or it might be a best practice for a specific case but by & large the idea is ludicrous.

The idea of managing the NPP also can not be done unless you have a perfectly air sealed house & can control the climate. One must remember that not only what the occupants do effects it, but so to does wind, solar, etc... - you couple that with "free" ventilation (i.e. pulling air in through insulation, mouse holes, squirrel nests, etc...) then you have lost any semblance of control, IAQ, and you are just throwing money away.

Along those lines you are also going to have condensation issues anywhere that the air is coming in as you are cooling down these sections so any warm humid air that hits that area where the dew point is reached is going to cause condensation & mold which will probably be hidden from site behind the drywall.

There is much resistance in the real world to running fans 24/7 even though some are small and very efficient. The idea of replacing construction flaws with well designed intentional openings really makes sense and can reduce the size or need for mechanical ventilation. Robert's examples certainly look like a step in the right direction, but I will need more time to consider the seasonal aspects and how to integrate passive venting with code requirements.

Sooner than we expect a smart box will be wanting to control all aspects of IAQ and understanding how we can take advantage of natural ventilation will be a positive contribution.

One to think on, a passive HRV. Even though its performance might me minimal, its cost would be low and acceptance high.

While I agree that "designing" a passive NPP is an exercise in futility, but designing an active interior pressure balance is an important consideration that is typically overlooked. An exhaust-only ventilation system maintains a more negative interior pressure balance (in relation to outside) and hence reduces dangerous cold weather exfiltration.

And, while we all agree that "natural" (uncontrolled) ventilation is NOT a rational option, there is no reason that Exhaust-Only Ventilation can not be a "best practices" approach. One: it's far less expensive both to install and to operate, so it will have wider acceptance. Two: some studies have shown that, with all operating costs considered, including electrical fan consumption and heating/cooling energy impacts, an exhaust-only system can be the least costly to operate - also giving them wider acceptance - while fully meeting ASHRAE IAQ requirements.

Coupling an exhaust-only whole-house fan (which typically is just one of the bathroom fans) with passive make-up air inlets makes it an even better "best practice" approach because it now controls at least most of the infiltration locations, eliminating or reducing that "mouse hole" air.

But you are incorrect, at least in a heating season, that "you are also going to have condensation issues anywhere that the air is coming in", as only exfiltrating air - going from warm to cold - can cause condensation. And condensation does not occur "where the dew point is reached" unless there is a condensing surface (like sheathing). Nor does mold and rot necessarily occur wherever there is condensation - only where the rate of wetting exceeds the rate of drying and there is insufficient moisture buffering.

LOL, I try Patrick - thanks for the reminder on follow ups (which I recommend every three to five years or when renovations will be happening) is I was back at one place & I noticed the ERV cap wasn't fully shut. Mind you this was through a sidewall & I knew the unit wasn't running at the time, so I popped the cap up to see what the issue was & there was this huge nest of wasps or hornets - oh what fun

Also very good solution and about the only thing one can add is to make sure you direct flow so it will blow out over the roof where it slopes down

OK! I think some of your questions depend on location of the house. Some depend on the type of construction. Some depend on the comfort issues in the home that are being addressed. There is no one right answer, because the house is a system.

Passive NPP - perhaps, but what about the 3 story home the major comfort issue is the stack temperature difference between 1st and 3rd. Using an H/E RV to put fresh air in high and exhaust low would lower the NPP and decrease the temperature difference.

Exhausting through a soffit. If we exhaust warm humid air into a cooler environment; it will rise. If that warm humid air bumps into an artificial ceiling (soffit) it will stop rising and start moving laterally. When it locates an opening into an attic, leak, opening, or soffit vent; it will enter the attic.

An exhaust vent carrying moist humid air leaves the bathroom, it can go up through the attic to the roof deck, or across the attic to the gable or soffit vent. In either case it is in the attic. If the ambient attic temperature is below its dew point the humidity will condense *** if the dew point in the attic is the same inside the duct ***. So insulating the duct raises the dew point inside the duct. If the humidity still condenses: the vertical duct will allow the condensation to run down the sides, into the fan housing and drip into the house. The homeowner has a problem and a clue that something is wrong. Condensation in a horizontal duct in an attic going to a soffit or gable will most likely pool at some point in the duct. You have a problem, no clue it is happening.

Since you have a powered air flow in the duct, it is best to have enough air flow to move the humid air through the duct to the outside. This calls for measurement of the actual flow on every exhaust fan. Knowing how long your duct is, you can easily figure the velocity requirements.

As with any intentional penetration of the shell, you must have an effective damper on the exterior. Most are not. Worth paying extra for. The exception would be the intake to an H/E RV that runs continuously.

Finally, the less penetrations of the roof, the better.

My ideal: Short straight run, to a gable, with insulated duct, buried in attic insulation, measured to provide enough air flow. Choice if gable is not available or practical; roof deck.

I think you nailed it John, though for long runs I would recommend that you pitch it to the outside so if any water does condense it drains out - in some cases the run is short enough that you don't need a fitting while in others you might need one offset or two 45's to make it